Vietnam Primate Conservation Status Review 2002 - Hoang Lien ...

Vietnam Primate Conservation Status Review Part 2: LEAF MONKEYS
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leaf parts. No animal remains were found in any of the stomachs examined (Pham Nhat, 1994b). One
should be cautious of a determination of diet from these data as some food sources are seasonal and so
diet may vary with food availability.
All four species of Rhinopithecus have a basic social unit of one-male / multifemale groups and in all
four species, these family groups associate in large bands of over 100 animals (in undisturbed
populations).
According to Boonratana & Le Xuan Canh (1994), R. avunculus form groups with an average group
size of 14.8 individuals. Other males form multi-male associations. No solitary animals were ever
observed. This social structure is flexible and two or more units often travel, sleep and eat together in
larger bands. This arrangement appears to be associated with large overlapping ranges among groups
(Boonratana & Le Xuan Canh, 1994). Ratajszczak et al. (1992) conducted interviews with local people,
which suggested that these large bands could reach more than 100 individuals. However, Ratajszczak
et al. (1992) considered the latter as the basic unit, which scatter when disturbed.
The Tonkin snub-nosed monkey is diurnal, arboreal, moves quadrapedally and by semi-brachiation.
It sleeps in the lower branches of trees and during the period of cold northeast winds, sleeping sites
are found close to steep mountain sides (Boonratana & Le Xuan Canh, 1994).
From distant observation, R. avunculus is generally silent (Ratajszczak et al.,1992). A distinct
vocalisation, sounding like a hiccup, may be heard if animals are alarmed or in general communication
within or between groups (Boonratana & Le Xuan Canh, 1998; R. Boonratana, pers. comm., 1998).
According to local hunters, after an animal is shot, the group generally does not move immediately
but rather remains motionless and silent. This behaviour makes these monkeys particularly vulnerable
to hunters, who can kill several animals at one time (Ratajszczak et al., 1992).
The sexual behaviour of R. avunculus is unknown. The extreme sexual dimorphism in body size and
canine dental complex for some species of snub-nosed monkeys has been interpreted by Jablonski
and Pan (1995) as a mechanism by which males compete for mates. That R. avunculus is the least
sexually dimorphic snub-nosed monkey could indicate that inter-male competition is less important
in this species.
5.0.3 Distribution
The zoogeographic range of each Rhinopithecus species is very restricted. R. roxellana (Golden snubnosed
monkey) is the most widely distributed, occurring in a series of disjunct forests around the
periphery of the Sichuan Basin (Sichuan, Gansu, Hubei and Shaanxi Provinces). R. brelichi (Guizhou
snub-nosed monkey) is restricted to a single population on Mount Fanjing (Guizhou Province). R.
bieti (Yunnan snub-nosed monkey) is located in five counties in Yunnan Province and Tibet.
All three Chinese species inhabit high mountain forest up to about 4,500m a.s.l. but may descend to
lower elevation in winter. Parts of their range are covered by snow for more than half a year.
The Tonkin snub-nosed monkey is endemic to northern Vietnam. Its range is historically limited to
areas to the east of the Red River. R. avunculus lives at relatively low elevations in subtropical
monsoon forests.